The present invention relates to ceramic matrix composites, and to methods for the manufacture thereof. More particularly, the invention relates to an improved method for manufacturing preforms for ceramic matrix composites wherein novel binder materials are used.
Ceramic matrix composites are well known. Typically they comprise a ceramic matrix material such as a glass, a glass-ceramic, or a crystalline ceramic material forming a matrix wherein refractory inorganic fibers are disposed as a reinforcing phase. A number of U.S. Pats. show the structure and manufacture of such composites, including, for example, U.S. Pat. Nos. 4,314,852, 4,588,699 and 4,615,987.
In conventional practice, composites incorporating ceramic matrix materials are manufactured from powders of the selected matrix or a matrix precursor, with the fibers being combined with the powders and consolidated to dense composite materials by the application of heat and pressure. To provide a homogeneous distribution of the fibers in the matrix the powders are generally first dispersed in a liquid vehicle. This liquid is then applied to the fibers as a coating or, in the case of short fibers, the fibers are added to the liquid and dispersed.
Conventional dispersions of matrix powders in liquid vehicles comprise, in addition to the powdered matrix itself, various dispersing agents, binders, and solvents which are needed to stabilize the dispersion and impart to the dispersion the properties of rapid drying and at least some flexibility after drying has occurred. Plasticizers to enhance flexibility may also be included.
Suspensions such as conventionally employed for these processes must be carefully formulated, handled, and stored in order to be effectively used. The powdered ceramics tend to settle in the suspensions and continuous mixing is normally required to maintain homogeneity therein. Stabilization of the suspensions against viscosity changes, which can occur during storage or use of these suspensions, may be required to avoid undesirable changes in the coating or fiber impregnation characteristics of the suspension.
Referring to specific ceramic suspensions known to the art, U.S. Pat. No. 4,412,854 discloses the manufacture of a fiber-reinforced composite preform from a suspension of a glass powder in vehicle comprising a thermoplastic polymeric binder, typically an acrylic polymer binder. The binder is dispersed or dissolved in a carrier liquid and powdered matrix material is added. The resulting liquid suspension is combined with a woven or non-woven fiber reinforcement which is then dried and optionally reshaped prior to consolidation.
U.S. Pat. No. 4,588,699 illustrates the use of an aqueous system wherein latex is employed as a binder, and also a solvent-based system wherein the matrix is simply dispersed in a liquid carrier of propanol.
More complex suspension systems are disclosed in U.S. Pat. No. 4,585,500, wherein a synthetic resin is dispersed in a solvent for the resin prior to the introduction of the powdered matrix material. The system further includes components such as paraffin wax, a plasticizer and a surfactant to act as dispersion and stabilization aids and to impart flexibility to the resulting preform. As in most matrix suspensions used for fiber-reinforced composites, the liquid components must generally be removed by evaporation from the fiber/matrix intermediate in order to provide a dry preform which can be handled, and plasticizing components are frequently helpful to retain at least some flexibility in the fiber/matrix assembly after drying.
Typical proportions of ingredients in these suspensions, as suggested by U.S. Pat. No. 4,581,053, are 20-50% by weight of matrix powder, 10-20% by weight of a polymeric binder, and the balance a carrier liquid. Components such as dispersing agents or the like are typically added in more minor amounts.
Notwithstanding the fact that a variety of different suspensions and techniques have been used for coating or otherwise combining reinforcing fibers with matrix materials, there remains a need for an improved coating system which will avoid the several problems of solvent-based systems presently being used for ceramic matrix composite manufacture. Improved suspension stability, better drying characteristics, and enhanced preform handling behavior are all needs which could be usefully addressed.
It is therefore a principal object of the present invention to provide improved matrix dispersions for the manufacture of ceramic matrix composites which will avoid certain of the shortcomings of prior art suspensions.
It is a further object of the invention to provide a method for manufacturing ceramic matrix composites which employs a dispersion of ceramic matrix powders in a thermoplastic vehicle to more effectively and efficiently impregnate or coat reinforcing fibers with such powders.
It is a further object of the invention to provide preforms or prepregs comprising combinations of ceramic matrix and reinforcing fiber materials, and methods making them, offering significant improvements in the handling and subsequent processing characteristics of the preforms or prepregs.
Other objects and advantages of the invention will become apparent from the following description.